An agreement by almost 200 nations to curb rising greenhouse gas emissions from 2020 will be far more costly than taking action now to tackle climate change, a new report says. Quick measures to cut emissions would give a far better chance of keeping global warming within an agreed UN limit of 2º Celsius above pre-industrial times to avert more floods, heatwaves, droughts and rising sea levels.

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"If you delay action by 10, 20 years you significantly reduce the chances of meeting the 2º target," said Keywan Riahi, one of the authors of the report at the International Institute for Applied Systems Analysis in Austria.

"It was generally known that costs increase when you delay action. It was not clear how quickly they change," he told Reuters on Wednesday (2 January) of the findings in the science journal Nature Climate Change based on 500 computer-generated scenarios.

It said the timing of cuts in greenhouse gases was more important than other uncertainties - about things like how the climate system works, future energy demand, carbon prices or new energy technologies.

The study indicated that an immediate global price of €15 a tonne on emissions of carbon dioxide (CO2), the main greenhouse gas, would give a roughly 60% chance of limiting warming to below 2º.